A laser beam that impinges on an optical element (e.g., an output coupler mirror of a laser resonator) is not entirely reflected or transmitted. Rather, a small portion of the laser beam is also absorbed by the optical element. Soiling of the optical element results in an increased absorption of the laser beam and therefore in heating of the optical element. With the high laser powers currently used, this can result in vaoprization of the respective optical element. The vapors produced by such vaporization are hazardous to health and pollute the surroundings (e.g., the laser resonator). In addition, a crack or break in an output coupler mirror with resulting vacuum leakage of a laser resonator can result in complete destruction of the laser.
German patent document DE 198 39 930 C1 discloses the use of an additional light source to monitor a protective glass. The radiation of the light source is coupled on one side of the protective glass, and the intensity of the radiation decoupled on the other, opposite side of the protection glass, where it is measured with a detector. Internal material defects of the protective glass such as cracks, can be detected by intensity changes. This monitoring apparatus is suited only for transparent optical elements. Moreover, the surface of the optical element cannot be monitored for functionality apart from the remaining optical element.
It is therefore desirable to improve the detection of functionality changes of optical elements.